(1) Field of the Invention
The present invention relates generally to high application-temperature coatings, and more particularly to a process of applying a protective, high-cure-temperature coating to thermally massive heat-sensitive aluminum alloy components without degrading the mechanical properties of the components.
(2) Description of the Prior Art
It has long been known and customary in the art to apply protective coatings on diverse types of structural substrates. Coating application methods generally include liquid dispersions of synthetic resins via surface application or immersion, or dry powder coating via electrostatic powder spray application. The choice of synthetic resins for use as a protective coating is dependent upon both the nature of the substrate to be coated and the nature of the intended application of the coated substrate. For effective protection, the chosen synthetic resin and application method must produce a continuous coating on the substrate of sufficient thickness and hardness to isolate the substrate from the environment(s) from which the substrate requires protection. Aluminum is an excellent selection for a structural material where high strength-to-weight ratios are desired/required. Heat treated six thousand (6000), seven thousand (7000) series and A356 aluminum alloys (containing 90-95% aluminum) are used in torpedo applications because of their high strength and relatively light weight. Traditionally, however, strength considerations are often given more consideration than corrosion resistance characteristics during design and material selection. Accordingly, the designed for strength-to-weight ratios are often undermined by corrosion when the chosen structural component is exposed to a marine environment for any length of time.
A variety of post-fabrication processes are currently used in an attempt to improve the corrosion resistance of torpedo structural elements made from series 6000, 7000 or A356 aluminum alloys. Anodizing, painting and chemical film conversion are the common surface treatments which improve corrosion resistance by isolating/separating the aluminum alloy from the corrosive environment. Powder epoxy coating is currently one of the most effective and practical surface coating/treatment that can be applied to an aluminum alloy.
The best corrosion resistance performance is generally obtained by using the highest-cure-temperature powder epoxies that are currently available. While small or thin aluminum parts can successfully be powder coated using standard commercial application techniques, "large thermally-massive" aluminum parts (parts with heavy thick-walled sections) cannot. Current methods of effectively applying such high-cure-temperature coatings to a thermally-massive part require that the part to be coated be heated in excess of temperatures at which the aluminum alloy begins to lose tensile/yield strength. Further, parts that are considered to be thermally massive act as a significant heat sink such that "normal" cold application of powder coatings followed by an oven cure are ineffective. During a cure cycle, the mass of a cold aluminum part draws so much heat away from the coating that it does not allow the coating a chance to flow before curing. A coating film applied this way would remain porous, discontinuous and rough. Note that there is no problem powder coating large steel components (which may be "large thermally-conductive masses") because steel is capable of withstanding much higher temperatures without degradation of structural properties.
For the purposes of this description, a "thermally-massive" aluminum component is defined as any component large enough or shaped in such a way as to draw heat away from the applied coating during a post spray cure. Some examples of thermally massive aluminum components as they apply to a torpedo structure are listed in Table 1 below.
TABLE 1 ______________________________________ Examples of Thermally Massive Aluminum Components Used in Torpedo Structures SECTION OR GENERAL SHAPE/ APPROX. WALL APPROX. DESCRIPTION DIMENSIONS THICKNESS WEIGHT ______________________________________ Ring 12" O.D. .times. 2.0" 10 lbs 8" I.D. .times. 1.5" long Hollow Cylinder 21" diameter .times. 0.35" to 39 lbs 12" long 2.12" Hollow Cone 21" diameter 0.3" to 41 lbs tapering to 1.5" 18.5" diameter .times. 15.5" long Hollow Cylinder 21" diameter .times. 0.3" to 58 lbs 12" long 2.0" Hollow Cylinder 21" diameter .times. 0.34" to 180 lbs 51" long 2.2" ______________________________________
In general, the components may be large or small, heavy or light, but typically have thick wall sections. The thick wall sections are slower to reach oven temperature and draw more heat away from surface applied coatings than the thinner sections.
The use of aluminum in torpedo components employs an abnormally large proportion of aluminum's strength in order to maximize torpedo effectiveness. Considering that torpedo components may be utilized for possibly 20 to 30 years, a multiple recoat capability must be accommodated in the torpedo coating application specification. During initial coating of newly manufactured hardware, recoats are occasionally required due to manufacturing error or other circumstances. This situation by itself generally requires the part to be subjected to additional full coating process cycles. In addition, if and when the component is severely damaged in fleet use and requires recoating, one or two additional coating cycles may be required at the torpedo maintenance depot. Since most of the torpedo hardware, after the production process, will not be tracked with respect to powder coating cycles, a high recoat capability must be assumed. Based on these circumstances, the maximum coating temperatures are generally set slightly lower to allow for multiple cycles. These safety factors have been built into the torpedo application specifications to protect the aluminum from excessive heating that could occur during production and subsequent depot repair.
Accordingly, to date, thermally massive series 6000, 7000 or A356 aluminum alloy components have not achieved high performance corrosion resistance characteristics because 1) a lower temperature (i.e., lower performance) coating is used, 2) a high-cure-temperature coating is used but applied/cured at temperatures that maintain the strength-to-weight ratio integrity of the component but that do not provide for sufficient curing of the coating, 3) a high-cure-temperature coating is applied/cured at its optimum temperature thereby degrading the mechanical properties of the component, or 4) multiple coating cycles necessitate the use of low cure temperatures to protect the structural integrity of the component.